Abstract not available

Bibliography: leaves 70-72 / vi, 73 leaves : ill ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Thesis (Ph.D.)--University of Adelaide, Dept. of Applied Mathematics, 1987

The objective of this study is to investigate and characterize the mechanism of the island wake behind an island called Liu Chiu Yu off the southwestern Taiwan coast based on the in-situ data of Sb-ADCP, CTD and satellite images. The findings suggest that a counter-clockwise eddy and a clockwise eddy both are with 0.01 S vortice appears in the wake of Liu Chiu Yu when the background flows are toward the northwest. The system of two eddies with opposite rotation and a central return flow develops an unsteady eddy shedding. On the other hand, when the background flows are toward the southeast, island wake generated in the lee of Liu Chiu Yu is attached system of two eddies with opposite rotation and a central return flow. The Sb-ADCP data shows that the flow pattern in the northeastern coast of Liu Chiu Yu is mainly semidiurnal. The major axis of the ellipse of the semidiurnal current is parallel to the orientation of the coast line (northeast to southwest) and the shape of the ellipse is quite long and narrow. The amplitude of the semidiurnal current is approximately two times that of the diurnal current. Generally, the currents are stronger and the occurring probability of the island wake is higher during spring tide. The CTD data shows that the eddy center appears to be divergent and upwelling occurs in the areas under the influence of island wakes. The upwelling pumps deep seawater to the surface and results in low temperature, high salinity, high oxygen concentration and low chlorophyll concentration. On the other hand, in the eddy edges, downwelling occur causing high temperature, low salinity, low oxygen concentration and high chlorophyll concentration. Strong shear was formed at the depth of 60m inside the island wake which generates thermocline so that the mixing phenomenon is quite obvious there. Moreover, in the regions without the influence of island wakes, the stratifying effect is clear and the horizontal variation of temperature, salinity and oxygen concentration is small. Therefore, neither upwelling nor downwelling occurs there. Furthermore, along the edge between blocking and free-stream areas, the shear stress increases and the mixing phenomenon arises to a certain degree. The satellite images show that an island wake appears in the southeastern Liu Chiu Yu during spring tide. The island wake develops a phenomenon called von Karman vortex street. At the same time, a counter-clockwise eddy with heavy suspensions appears in the northern Liu Chiu Yu. The radius of the eddy is around 4 Km. The area of the lowest chlorophyll concentration is located at the center of the eddy. By analyzing all these data, it is concluded that the island wake in Liu Chiu Yu usually appears during spring tide.

The flow past the needle of a Pelton turbine injector is an axisymmetric wake embedded in a round jet. The wake does not fully relax to yield a uniform velocity jet due to the short distance between injector and the Pelton wheel buckets and this non-uniformity affects the turbine efficiency. To minimize the non-uniformity, it is essential to predict the near wake accurately. While far-field wakes are well described by analytical expressions and also well predicted by CFD codes, the quality of the prediction of axisymmetric near wakes is not known. It is of practical interest to establish the applicability bounds of the Reynolds Averaged Navier-Stokes (RANS) models, which are commonly used in industry, for axisymmetric near wakes, for this specific problem, as well as, in general. Understanding of the near wake is crucial considering various aerospace applications. For example the details of the aerodynamics of the near wake are crucial for stabilization of a flame. The size of recirculation zone affects the rate of production of hot burnt products, and the mixing between the products and reactants is governed by the turbulence in the free shear layers. Wakes from two-dimensional bodies such as a wedge, circular and square cylinder have been extensively studied at different Reynolds number (Re); however, this is not the case with three-dimensional axisymmetric bodies such as spheres, ellipsoids, disks etc. Most common axisymmetric body investigated is a sphere. The flow past sphere is typically characterized in three regions: sub critical, critical and supercritical. In sub critical region, Re<3x105 the boundary layer separation is laminar. Critical region, Re≈3x105, is where the boundary layer transitions to turbulent and then separates resulting in sudden drag reduction. The critical Re may vary depending on flow conditions such as turbulent intensities, sphere surface variations etc. In the supercritical region, Re > 3x105, the boundary layer is turbulent before separation and the drag starts increasing beyond critical drag. Though the geometry and the flow conditions are simple the flow features involved are complex especially laminar to turbulent boundary layer transition and high speed transient vortex shedding. Experimentally it has been observed that the vortex shedding location changes randomly and perhaps rotates. All these features pose a significant challenge for experimental measurements and as well as numerical modeling. Thus most experimental measurements have been done below Re=103. Also the data is measured over the sphere surface, for eg: skin friction, pressure, but almost no data is available in the near wake. Similarly numerical investigations are primarily in subcritical region. DNS has been used for low Re, up to 800. RANS has been used in the subcritical region at Re=104. For higher Re, LES and DES have been used however they are computationally intensive. No numerical work has been reported for an ellipsoid at zero angle of attack. Chevray (1968) has done measurements in the near wake of ellipsoid at Re=2.75x105. Most experimental and numerical investigations of an ellipsoid are at an angle of attack. Given the extensive usage of RANS in the industry due to its economy, the focus of this work is to investigate the applicability of these models for flow prediction in the near wake in the supercritical region. Simulations are performed using commercial code CFX. The code is validated against well-established results for laminar and turbulent boundary layer flow over flat plate. Sufficient agreement has been obtained for laminar flow past sphere, against measured quantities such as separation location, separation bubble length and drag coefficient. The changes in wake structure, as a function of Re, are validated against experimental observations. The wake is steady and axisymmetric up to Re=200, from Re=200 to 270 it remains steady, loses axisymmetry but retains planar symmetry. Beyond Re=290 the wake becomes unsteady due to unstable recirculation bubble which leads to vortex shedding, while still retaining planar symmetry. The formation of typical horseshoe vortices is observed. Before the simulations in the supercritical region the low-Re k- model is validated in the subcritical region at Re=104 against measurements of skin friction, pressure coefficient and average drag coefficient. Very distinct wake fluctuations are observed and low-mode Strouhal number (St) agrees with the past measurements. Vortex sheet fluctuations are observed but the high-mode St calculation is based on crude measurement of the fluctuations. At Re=7.8x104 the trends in the drag, skin friction coefficient and pressure coefficient are in logical direction when compared with data at Re=104. However the near wake velocity data does not match with measurements qualitatively as well as quantitatively. The velocities in the present work are qualitatively justified based on the flow directions in the recirculation bubble. Various RANS models such as k-, k- and Reynolds stress model are used to predict flow past a sphere and an ellipsoid in the supercritical region. The results for sphere are compared against the measurements from Achenbach at Re=1.14x106 and that for ellipsoid are compared against the measurements from Chevray at Re=2.75x106. Four different turbulence models namely: high-Re k-, high-Re k-, low-Re k- and low-Re RSM. All the models over predict skin friction, which is due to simplistic treatment of boundary layer. The boundary layer is treated as fully turbulent as against the experiments where it transitions from laminar to turbulent. The k- model, being high-Re model, did not capture near wall flow and hence predicts an almost steady wake. It over predicts the drag, skin friction and results in delayed separation. However it did show the vortex sheet roll-up and release mechanism prominently which agreed with the experiments by Taneda. In all other models this mechanism is seen but intermittently and the wake is unsteady. Due to highly random wake orientation the low-mode St number is not calculated. RSM model shows certain consistency and St based on that is 0.24. All models show vortex sheet fluctuations with almost equal magnitude and frequency. The high-mode St is about 20 based on this. There is a need to have better understanding both experimentally and numerically about validity of this number. High frequency fluctuations are displayed in the time history of streamwise drag force for all the four models. The St based on this frequency is 4.32. Origin of these fluctuations needs investigation. The RSM model predicts the most accurate skin friction coefficient, pressure coefficient and the drag. For an ellipsoid, two cases are computed, one without blockage (referred to as base case) and another with 25% blockage (referred to as blockage case) to represent the typical blockage due to Pelton injector needle. Same models that were used for sphere are evaluated. Similar to the results for the sphere the maximum drag is predicted by k- model and the least by RSM model. Similarly the skin friction is high and the separation is delayed hence k-w model always predicts a smallest recirculation bubble. The differences in the form drag predictions are a direct result of the differences in upstream stagnation pressures, as there is no significant difference in the pressure curves obtained from different models including the rear stagnation pressure. The form drag is highest in k- model and lowest in RSM and so are the upstream stagnation pressures. The velocities in the near wake are predicted well by all the models. Pressure is predicted accurately before separation at x/D=-0.25. However it is significantly over-predicted after separation. To validate the pressure prediction independent simulation is done for an ellipsoid at an angle of attack of 100. The pressures on the windward and leeward side are in agreement with the measurements by Chesnakes et al. Similar to pressure prediction the turbulent intensity was predicted correctly before separation. After separation the trends agree but the intensities are higher than the measurements by about 10%. The results are not sensitivity to the inlet intensity levels except in the far field. The dissipation of the intensities is under predicted in simulations. The results from blockage case show similar trends as the base case. In the near wake the generation of turbulent kinetic energy is higher and the decay is slower in k- and RSM model compared to k-. This in turn results in higher eddy viscosity and higher velocities in the near wake for these models. Considering overall prediction accuracies RSM model predicts the drag, St and the separation location most accurately. It is important to predict the separation accurately for valid downstream results. For the cases with mild separation such as ellipsoid there is no significant difference in the velocities, however the pressure and drag prediction from RSM are closer to the experiments. The RSM model is more suitable both for sphere and ellipsoid at high Re. Validation of mean velocities and intensities in the near wake are needed to further support the choice of model. (for symbols pl see the original document)

No description available.

Combustion instability plagues the combustion community in a wide range of applications. This un-solved problem is especially prevalent and expensive in aerospace propulsion and ground power generation. The challenges associated with understanding and predicting combustion instability lie in the flame response to the acoustic field. One of the more complicated flame response mechanisms is the velocity coupled flame response, where the flame responds dynamically to the acoustic velocity as well as the vortically induced velocity field excited by the acoustics. This vortically induced, or hydrodynamic, velocity field holds critical importance to the flame response but is computationally expensive to predict, often requiring high fidelity CFD computations. Furthermore, its behavior can be a strong function of the numerous flow parameters that change over the operability map of a combustor. This research focuses on a nominally two dimensional bluff body combustor, which has rich hydrodynamic stability behavior with a manageable number of stability parameters. The work focuses first on experimentally characterizing the dynamical flow and flame behavior. Next, the research shifts focus toward hydrodynamic stability theory, using it to explain the physical phenomena observed in the experimental work. Additionally, the hydrodynamic stability work shows how the use of simple, model analysis can identify the important stability parameters and elucidate their governing physical roles. Finally, the research explores the forced response of the flow and flame while systematically varying the underlying hydrodynamic stability characteristics. In the case of longitudinal combustion instability of highly preheated bluff body combustors, it shows that conditions where an acoustic mode frequency equals the hydrodynamic global mode frequency are not especially dangerous from a combustion instability standpoint, and may actually have a reduced heat release response. This demonstrates the very non-intuitive role that the natural hydrodynamic flow stability plays in the forced heat release response of the flame. For the fluid mechanics community, this work contributes to the detailed understanding of both unforced and forced bluff body combustor dynamics, and shows how each is influenced by the underlying hydrodynamics. In particular, it emphasizes the role of the density-shear layer offset, and shows how its extreme sensitivity leads to complicated flow dynamics. For the flow-combustor community as a whole, the work reviews a pre-existing method to obtain the important flow stability parameters, and demonstrates a novel way to link those parameters to the governing flow physics. For the combustion instability community, this thesis emphasizes the importance of the hydrodynamic stability characteristics of the flow, and concludes by offering a paradigm for consideration of the hydrodynamics in a combustion instability problem.

CD-ROM containing source codes of the numerical scheme (appendix A) is attached to back cover. / Includes bibliographical references (leaves 459-472). / System requirements for accompanying CD-ROM: Macintosh or IBM compatible computer. Other requirements: Adobe Acrobat Reader. / xxxvi, 473 leaves ; ill. ; 30 cm. + 1 computer optical disk (4 3/4 in.) / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / A numerical scheme, based on discrete-vortex and surface-vorticity boundary-integral methods, has been developed for stimulating time dependent, two-dimensional, viscous flow over arbitary arrays of solid bodies of arbitary cross-section / Thesis (Ph.D.)--Adelaide University, Dept. of Mechanical Engineering, 2001

Thesis(M.S.)--Auburn University, 2007. / Abstract. Vita. Includes bibliographic references.

Thesis (M.Oc.E.)--Oregon State University, 2011. / Printout. Includes bibliographical references (leaf 70). Also available on the World Wide Web.

Les amplifications optimales d'énergie de structures quasiment alignées dans le sens de l'écoulement sont calculées dans le cas d'un sillage parallèle, d'un sillage synthétique faiblement non-parallèle et du sillage d'un cylindre. Il a été observé que de très grandes amplifications d'énergie peuvent être supportés par ces sillages. L'amplification d' énergie s'accroît avec la longueur d'onde des perturbations en envergure à l'exception du sillage du cylindre pour lequel l'accroissement d'énergie est maximal pour λz ≈ 5 − 7 D. Les structures amplifiées de manière optimale sont les streaks fluctuant dans le sens de l’écoulement. Il est montré que ces streaks sont capables de supprimer complètement l'instabilité absolue d'un sillage parallèle lorsqu'ils sont déclenchés avec une amplitude finie. L'instabilité globale d'un sillage faiblement non-parallèle et celle du sillage d'un cylindre peuvent être complètement supprimées par des streaks d'amplitude modeste. L'énergie de contrôle requise pour stabiliser le sillage est très faible lorsque les perturbations optimales sont utilisées, et il est montré qu'elle est toujours plus faible que celle qui devrait être utilisée pour un contrôle uniforme en envergure (2D). Il est aussi montré que la dépendance du taux de croissance est quadratique et que, par conséquent, les classiques analyses de sensibilité au premier ordre ne permettent pas de prédire la grande efficacité de la technique de contrôle par streaks. La dernière partie de ce travail livre des résultats préliminaires sur l'étude expérimentale du contrôle par streaks dans le cas du sillage turbulent d'un corps 3D. Il est montré que les streaks forcés artificiellement dans la zone d'instabilité absolue de l'écoulement sont capables de modifier la dynamique du sillage. / We compute optimal energy growths leading to streamwise streaks in parallel, weakly non-parallel and the circular cylinder wakes. We find that very large energy amplifications can be sustained by these wakes. The energy amplifications increase with the spanwise wavelength of the perturbations except in the circular cylinder wake where maximum energy growths are reached for λz ≈ 5 − 7 D. The optimally amplified structures are streamwise streaks. When forced with finite amplitudes these streaks are shown, in parallel wakes, to be able to completely suppress the absolute instability. The global instability of the weakly non-parallel and the circular cylinder wakes can be completely suppressed with moderate streaks amplitudes. The energy required to stabilize the wake is much reduced when optimal perturbations are used, and it is shown to be always smaller than the one that would be required if a 2D control was used. It is also shown that the sensitivity of the global mode growth rate is quadratic and that therefore usual first order sensitivity analyses are unable to predict the high efficiency of the control-by-streaks strategy.